A strong knowledge of the basic multiplication facts helps students immensely later in school. 4th graders should be working on memorizing their multiplication facts up through 12 X 12. It wouldn't hurt to go as far as 15 X 15 or even 20 X 20.
For fourth graders, they should know up to their 12 times tables, the highest multiplication fact being twelve times twelve. Helping your son memorize these facts is a very good idea, make sure to practice every day!
A fourth grader should normally have a good handle on all of the multiplication tables up through 12x12. The best way for him to learn is to keep practicing over and over until he finally is able to memorize all of them. Start with the ones, and go through 1x1 up through 1x12.